A Wall‐Like Sharp Downward Branch of the Walker Circulation Above the Western Indian Ocean
A Wall‐Like Sharp Downward Branch of the Walker Circulation Above the Western Indian Ocean
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DOI：
10.1029/2021jd034650
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发表时间：
2021-02
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影响因子：
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通讯作者：
T. Kohyama;T. Suematsu;H. Miura;Daisuke Takasuka
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作者：
T. Kohyama;T. Suematsu;H. Miura;Daisuke Takasuka
在纬向上，印度洋上空步行者环流的向下分支只有20°宽，而太平洋环流的向下分支则有90°宽。这种纬向清晰度值得注意，因为小于行星尺度的大气扰动可以通过该分支与行星尺度的步行者环流相互作用。这种纬向的尖锐性也印刻在非洲东北部上方热带雨带独特的纬向间断上。因此，我们把这个狭窄的向下分支称为“墙”，研究它的气候学和年际变化，并旨在确定它存在的原因。在北半球夏季对流层低层壁的最强季节是由与亚洲夏季风相关的水平冷平流维持的，而在北半球春季的另一个峰值则是由纬向风的季节性迁移解释的。墙的两个弱相对应于非洲之角东部的两个雨季，最先进的全球气候模式无法很好地再现。使用允许对流的大气模型的实验表明，东非山波强迫的垂直混合对于维持墙是必要的。东非地形变平后，热带雨带的纬向不连续性消失。对于年际变率，长城垂直运动的一个标准差变率与热带太平洋海表温度的一度相关，并且这种关系在北方秋季最强。然而，总的方差更多地是由热带印度洋的海面温度来解释的。
In the zonal direction, the downward branch of the Walker circulation above the Indian Ocean is only 20° wide, whereas the Pacific counterpart is 90° wide. This zonal sharpness is notable because atmospheric disturbances smaller than the planetary scale can interact with the planetary‐scale Walker circulation through this branch. This zonal sharpness is also imprinted on a unique zonal discontinuity of the tropical rain belt above Northeast Africa. Therefore, we refer to this narrow downward branch as the “Wall,” investigate its climatology and interannual variability, and aim at determining its reason for existence. The strongest season of the lower tropospheric Wall in boreal summer is sustained by horizontal cold advection associated with the Asian Summer Monsoon, whereas another peak in boreal spring is explained by the seasonal migration of zonal winds. Two weak phases of the Wall correspond to two rainy seasons at the Eastern Horn of Africa, which are not reproduced well by the state‐of‐the‐art global climate models. Experiments using a convection‐permitting atmospheric model show that vertical mixing forced by mountain waves in East Africa is necessary for sustaining the Wall. After flattening the East African topography, zonal discontinuity of the tropical rain belt disappears. As for interannual variability, one standard deviation variability of vertical motions at the Wall is associated with one degree of sea surface temperature in the tropical Pacific, and the relationship is strongest in boreal autumn. Nevertheless, total variance is explained more by sea surface temperature in the tropical Indian Ocean.